Erika Enterprise is a free of charge and open-source RTOS, OSEK/VDX certified.
RT-Druid is a tool for architecture level design of embedded systems.
SMCube (or SM3) is a State Machine System Modeler, a software written in C++ that allows, through a friendly graphical interface, to design and create applications (in C or Verilog) based on Finite State Machine (FSM).
E4Coder is a set of tools that can be used to simulate control algorithms and to generate code for embedded microcontrollers running with or without a realtime operating system.
Integrated Eclipse Forms GUI Designer, Builder, and Code Generator (demo available here)
Scicos is a modeler/simulator for hybrid dynamical systems.